ELECTRICALLY-EVOKED CATECHOLAMINE RELEASE FROM CAT ADRENALS - ROLE OF CHOLINERGIC RECEPTORS

Catecholamine (CA) release from perfused cat adrenal glands was continuously monitored using an on-line system coupled to an electrochemical detector. This highly sensitive procedure allowed the detection of small changes in the rate of secretion, even using short trains of electrical stimulation or brief acetylcholine (ACh) pulses. CA release was linear with increasing strength of ACh, transmural or splanchnic nerve stimulation. By using specific blockers, the contribution of nicotinic or muscarinic receptors to the overall secretory response to various stimuli could be established. That nicotinic receptors play a major role in mediating the secretory response to all stimuli is shown by the clear inhibition of the response with mecamylamine (10-mu-M). In contrast, atropine (1-mu-M) halved secretion evoked by ACh or nerve stimulation but had little effect on the response to trains of transmural electrical stimulation. When transmural electrical stimulation was applied continuously (instead of in trains), increasing the frequency in a step-wise manner, a bell-shaped curve was obtained; secretion reached a peak at 8 Hz and then declined sharply at 16 and 32 Hz. With this stimulation pattern, atropine decreased by 50% the secretion response at the higher frequencies (4-32 Hz). Very few studies are available which define the role of receptors and ionic channels in mediating electrically-evoked CA release. These stimulation patterns have not been used previously and are likely to mimic more closely than those used in earlier studies the physiological firing pattern of splanchnic nerves innervating adrenomedullary cells.